The present invention relates to a tensioner for a traction member, in particular a belt of a traction drive.
Tensioners of a type involved here are used in particular in internal combustion engines for motor vehicles to ensure a slip-free drive of accessories or units that are propelled by the traction drive during start-up operation as well as generator operation, without affecting the speed pattern of the traction drive. The traction drive is hereby operated off the engine crankshaft and connects pulleys of the accessories that need to be propelled. Oftentimes, internal combustion engines include two traction drives, whereby one traction drive represents the control mechanism for driving the camshaft(s), and the other traction drive is used as accessory drive for operating, e.g., a water pump, injection pump, air conditioning compressor, servo devices and other accessories. As a consequence of an increase in length of the traction member and an uneven rotation of the crankshaft, caused by the combustion process of the internal combustion engine, dynamic effects, such as vibrations of the traction member, are encountered. Moreover, the material of the traction member undergoes a temperature-based change so that the biasing force of the traction member, i.e. the belt tension, changes especially at high or low limit temperatures. Due to aging and wear, belts have a tendency to stretch so that the initially set tension of the traction member decreases.
German patent publication no. DE 198 45 710 A1 describes a tensioner having a base which is fixedly mounted to a machine part. A tensioner arm includes a hub which is supported on a pin of the base for connection of the tensioner arm via a shaft with the base. With its free, the tensioner arm is resiliently supported upon the traction member via a rotatable tension roller. In the area of the shaft, one end of the pin carries a radial flange, with a friction lining or friction disk disposed between end surfaces of the radial flange and the hub of the tensioner arm and operating as damping element to dampen an adjusting movement of the tensioner arm.
The tensioner according to German patent publication no. DE 43 43 429 A1 is detachably secured to a housing of an internal combustion engine. A shaft is provided between the base and the tensioner arm and surrounded by a torsion spring having one spring end secured to the base and another spring end secured to the tensioner arm. The torsion spring surrounds hereby a central portion of the base for swingable support of the bearing pin of the tensioner arm via a bearing in the form of a slide bearing sleeve. The tensioner arm is axially supported by the base via a radial flange which is disposed on one end of the bearing pin and supported by an end face of the base through intervention of a friction lining which further assumes the function of a damping element. The free end of the tensioner arm carries a rotatable tension roller which resiliently rests against the traction member.
Conventional tensioners are dimensioned for a maximally transmittable torque of the accessories, e.g. 100% generator load. Apart from the fact that the life of the traction member is adversely affected thereby, there are other negative consequences. Because conventional tensioners are dimensioned for maximum load conditions, an excessive biasing force is applied, even though the accessories may operate at reduced load, e.g. during idling of the internal combustion engine. In other words, the required biasing force is less than the actually applied biasing force, rendering the traction drive overly taut. On the other hand, when the biasing force is too low at high moments, an increased slip between the traction member and the pulleys is encountered. Thus, conventional tensioners are constructed to attain a compromise in order to reconcile the dynamic effects and the slip, on the one hand, and the noise development, on the other hand.
It would therefore be desirable and advantageous to provide an improved tensioner which obviates prior art shortcomings and which allows a continuous adjustment of the biasing force and thus tension of the traction member to the load of the accessory and the operating phase of the internal combustion machine, regardless of the speed pattern, i.e. to reduce the biasing force at high rotation speed and to increase the biasing force at low rotation speed.